1. Field of the Invention
This invention relates to an electrostatic actuator and a method of driving the same. More specifically, the present invention relates to an electrostatic actuator that does not comprise any structural bearing nor guide and is compact and adapted to support the rotor in a non-contact way and drive it in a desired direction of rotation with a low voltage.
2. Related Background Art
Various electrostatic actuators have been proposed in recent years. The electrostatic actuator provides the following advantages relative to the electromagnetic actuator.
(1) It is highly efficient and does not give rises to any loss in a stationary state.
(2) Since it comprises only electrodes, it is structurally simple and adapted to down-sizing.
FIG. 1 of the accompanying drawings is a schematic perspective view of a floating rotary drive system using an electrostatic actuator that was proposed at page 537 of the released papers for the lectures in the 1998 Spring Meeting of the Japan Society for Precision Engineering. The floating rotary drive system uses an electrostatic actuator comprising a rotor 401 and a stator 402 and is designed to eliminate the frictional force between the rotary 401 and the stator 402 by causing the rotor 401 to acoustically float relative to the stator 402. FIG. 2 schematically illustrates the pattern of arrangement of the electrodes on the stator 402 of the electrostatic actuator of FIG. 1.
Referring to FIGS. 1 and 2, the oscillator 404 is driven to operate according to a signal from oscillator driver 405. With this electrostatic actuator, an acoustic floating force is generated on the surface of the stator 402 as the oscillator 404 is driven to operate and the oscillation of the oscillator 404 is amplified by means of a horn 403 so that the rotor 401 is supported above the surface of the stator 402 in a non-contact way. Thus, the rotor 401 is driven to rotate as a drive voltage is applied to drive electrodes 406 according to the signal from electrostatic drive circuit 407. The drive voltage output from the electrostatic drive circuit 407 is a 3-phase AC voltage. The rotor is made of glass.
As shown in FIG. 2, a large number of circular electrodes 406 are arranged on the stator 402 so that both a torque and a centripetal force arise simultaneously and are applied to the rotor 401. The drive voltage is 1 kV p-p and the rotor 401 rotates only clockwise.
The above described known electrostatic actuator is, however, accompanied by the following problems. Firstly, the rotor of the electrostatic actuator rotates only clockwise. Secondary, the electrodes of the stator have a circular profile and are separated from adjacent ones by a large distance so that the inductive electric charge generated on the rotor is located remote from the electrodes on the stator to consequently reduce the rotary force that is actually applied to the rotor. In other words, a large drive voltage is required to make the electrostatic actuator operate properly. Then, the overall arrangement will be rather bulky.
In view of the above identified problems of the prior art, it is therefore the object of the present invention to provide an electrostatic actuator that is compact and adapted to drive the rotor in a desired direction of rotation with a low drive voltage and a method of driving such an electrostatic actuator.
According to the invention, the above object is achieved by providing an electrostatic actuator comprising:
a stator formed by a circular substrate having a surface carrying thereon a plurality of drive electrodes, each of the drive electrodes having an arcuate profile extending from the center of the substrate toward the outer periphery thereof;
a rotor arranged opposite to the surface of the stator carrying thereon the drive electrodes; and
a drive circuit for applying a drive voltage to the drive electrodes of the stator.
According to the invention, there is also provided a rotary stage comprising:
an electrostatic actuator as defined above; and
a sample holding jig arranged on the rotor of the electrostatic actuator.
According to the invention, there is also provided an optical scanning device comprising:
a light source adapted to emit a laser beam;
an electrostatic actuator as defined above; and
a polygon mirror arranged on the rotor of the electrostatic actuator and adapted to deflect the laser beam emitted from the light source.
According to another aspect of the invention, there is also provided a method of driving an electrostatic actuator comprising a stator formed by a circular substrate having a surface carrying thereon a plurality of drive electrodes, each of the drive electrodes having an arcuate profile extending from the center of the substrate toward the outer periphery thereof, a rotor arranged opposite to the surface of the stator carrying thereon the drive electrodes, a drive circuit for applying a drive voltage to the drive electrodes of the stator and an oscillator for generating dynamic pressure between the stator and the rotor, the method comprising the steps of:
generating dynamic pressure between the stator and the rotor by means of the oscillator to float the rotor above the stator; and
applying a multiphase drive voltage to the drive circuit to drive the rotor to rotate.